A) Field of the Invention
The present invention relates to a solid state image sensing apparatus and more particularly to a solid state image sensing apparatus with an exposure sensitivity adjusting function.
B) Description of the Related Art
CCD and MOS solid state image pickup devices are prevailing which are used as solid state image pickup devices for digital cameras. Photodetectors of both the CCD and MOS solid state image pickup devices are made of photodiodes. In a general solid state image pickup device, a light shielding film with registered openings is disposed above photodiodes, color filters are disposed above the light shielding film, and on-chip micro lenses are disposed above the color filters in register with the respective photodiodes.
In a CCD solid state image pickup device, a charge coupled device (CCD) is disposed near a photodiode column to transfer charges accumulated in photodiodes of that column. In an area sensor, photodiodes are disposed in a matrix shape and each photodiode column is coupled to a vertical CCD (VCCD). One ends of a plurality of VCCDs are coupled to a horizontal CCD (HCCD) to transfer image charges of each row to an output amplifier.
In a MOS solid state image pickup device, a MOS charge detector is connected to each photodiode and supplies a voltage or current output signal corresponding to a light reception amount to an output signal line at each column.
It is necessary to increase the number of pixels in order to improve the resolution of a solid state image pickup device. As the number of pixels is increased by making each photodiode finer, the area occupied by each photodiode reduces so that the photographic speed or sensitivity lowers. In order to maintain or increase an output level, it is necessary to raise the sensitivity of an output amplifier of a CCD solid state image pickup device or a charge detector of a MOS solid state image pickup device. An output level of a solid state image pickup device can be improved, for example, by reducing the capacitance C of a capacitor (floating diffusion capacitor) for converting charges into voltage.
CCD solid state image pickup devices have been developed which have, for example, an output level (saturated output) of 3000 mV and a detection sensitivity of 48 μV/e. An output of a solid state image pickup device is supplied to a signal processor circuit formed in another chip whereat the output is subjected to correlated double sampling, analog amplification, analog/digital (A/D) conversion and the like. An input dynamic range of the signal processor circuit is at most about 1 V.
A solid state image sensing apparatus having a sensitivity setting function can set a photographic speed or sensitivity such as ISO100, ISO400, ISO800 and ISO1600. The gain of a signal processor circuit is adjusted in accordance with the photographic sensitivity. As the signal level becomes high, the noise level also becomes high. In order to form a good image, a wide dynamic range and a low noise level are desired.
In a solid state image sensing apparatus having a solid state image pickup device and a signal processor circuit, both the solid state image pickup device and the signal processor circuit are provided with an amplification function.